1. Technical Field
This invention relates to methods and machines for manufacturing rotors from forging and, more particularly, to pre-spinning rotor forgings to relieve forging induced residual stresses.
2. Background Information
During normal operations gas turbine engines may operate with high rotational speeds and relatively high temperatures. Residual stresses from a metal alloy forging process used in fabricating turbine disks in the engine may relieve during engine operation, such that the turbine disks may undesirably expand. Such disk expansion may adversely affect clearances between the rotor and surrounding casing during engine operation.
In order to reduce occurrences and degree of disk expansion, at least some known engine disks are spun during the manufacturing process in a near-finished condition to relieve the residual stresses in the disk or other rotor forging. This process is known as pre-spinning of the disks generally has the same effect on relieving the residual stress as actual engine operation. Final machining, such as, of mating or other features and/or rabbets, for example, is performed after the pre-spinning process. Conventional pre-spinning processes is a time consuming and costly process. Moreover, because the high rotational speeds are needed to relieve the residual stresses the rotor forging and its mounting fixture must be balanced which increases the complexity of the pre-spin process.
Pre-spin machines provide the capability to spin rotating parts at speeds exceeding aircraft engine core speeds which are about 13,000 RPM. In order for the machine to handle to amount of force generated by the rotating parts unbalance, the rotating part, fixture, alignment, and spindle must be balanced to values typically less than 500 gram-inches using separate balance machines. Pre-spin machines are designed to be operated in a speed range of about 5,000 RPM to 18,000 RPM and balance machines between about 100 RPM and 1200 RPM. Pre-spin machines are intended to hold the part solidly and are typically of the soft-bearing type which means the vibration response changes with increasing speed. Balance machines roll the parts on “knife-edge” bearings and try to maximize unbalance sensitivity within the pedestals holding the bearings. Pre-spin machines are not designed to balance a part, typically they just spin parts to high speeds and are designed to spin parts fast enough for the part to actually fail.
Even if the rotating part and fixture can be balanced separately in a stand-alone balance machine, they must still be aligned properly in the pre-spin machine. If not, reaching proper high speeds may not be attainable. This problem then creates significant cost prior to even pre-spinning the rotating part. Costs may include machining of rotating part to tight tolerances, balancing fixture, properly aligning fixture, and properly aligning pre-spin spindle. If unbalance is still too high, process may have to be repeated many times to be able to achieve pre-spin speeds.
Another problem relates to the wear of the pre-spin machine over time for rotating parts still having high vibration levels. This may cause premature failure of specific components or excessive preventative maintenance costs to prevent these failures. Failure within the pre-spin machine can cause significant damage to the rotating part, fixture, spindle, and machine. Yet another problem with having high unbalance and vibration in the pre-spin machine is the potential for the rotating part to shift within the fixture causing higher vibration during deceleration than during the acceleration to top pre-spin speed. This concern can also cause premature machine failure and rotating part damage.
Thus, it is highly desirable to provide apparatuses and methods that are less expensive and less time consuming machine then conventional machines and methods for balancing and pre-spinning a rotor forging to relieve forging induced stresses at rotational speeds about or above maximum operational speeds of a rotor part manufactured from the rotor forging.